Oleophobic fluoropolymers and filter materials prepared therefrom

ABSTRACT

The disclosure provides an improved process for preparing oleophobic fluoropolymers. The resulting fluropolymers were found to be effective in imparting hydrophobic and oleophobic properties to porous polymeric membranes such as PTFE. The treated PTFE membrane can reach an oleophobic level of 6 or above, (according to the AATCC-118-1997 oil repellency test method) with a low air flux loss.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application under 35 U.S.C. 111(a)claiming priority under 35 U.S.C. 120 to International Application No.PCT/CN2021/102859, filed Jun. 28, 2021, the disclosure of which ishereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to methodology for preparing oleophobic polymers,which are useful in imparting oleophobic coatings to filter materials,such as those comprised of porous membranes.

BACKGROUND

Most of the housings of electronic equipment such as automobile lights,electric razors, and electric toothbrushes are designed with vents. Thepurpose of the vent is to extend the service life of the equipment byeliminating the internal and external pressure imbalance caused bythermal expansion and contraction inside the equipment by balancing theinternal and external atmospheric pressure. In order to prevent water,dust, oil vapor, etc., from entering through the vents of the equipmenthousing, poly(tetrafluoroethylene) microporous membranes (PTFEmembranes) are often used as venting filter materials.

The PTFE membrane itself has excellent hydrophobicity. However, in someapplications, the breathable membrane at the vent will inevitably be incontact with low surface tension substances such as lipids, surfactants,and oils. In these circumstances, a PTFE membrane alone often cannotprevent the undesired intrusion of such substances. Accordingly, thereis a need to achieve enhanced oleophobic and hydrophobic properties forventing membranes, such as PTFE venting membranes.

It is known that fluoropolymers which contain a linear perfluoroalkylgroup with 8 or more fluorocarbon atoms (C8) can impart excellent waterand oil repellency to materials. Existing synthetic methodology includes(1) a method of obtaining a fluorine-containing acrylate polymer byemulsion polymerization and (2) a method of preparing a fluoropolymervia a polycondensation reaction between polyisocyanate groups andfluorine-containing monomers with polyhydroxyl groups (See, for example,U.S. Pat. No. 5,342,434). However, one difficulty is that with suchfluoropolymers having sufficiently high fluorine content so as to impartgood oleophobic and hydrophobic properties also suffer from poorsolubility in non-fluorinated solvents.

Additionally, due to the high inherent hydrophobicity of the PTFEmembrane, existing fluorine-containing polymer emulsions cannotcompletely wet the PTFE membrane; consequently, the PTFE membrane cannotbe modified uniformly and effectively (i.e., coated) to increase theoleophobicity of the membrane. Moreover, the polymerization reactionbetween isocyanate group(s) and fluorinated monomer(s) with hydroxylgroup(s) is difficult to scale up, since the reaction is typically abulk polymerization and the reaction product tends to adhere to thereaction wall and the stirring blade, thus making it difficult toefficiently remove and isolate the reaction product from the reactor.Additionally, isocyanate-based compounds present other concerns such astoxicity and potential damage to the environment.

SUMMARY

The present disclosure provides an improved process for preparingoleophobic fluoropolymers. Advantageously, the process can be readilyscaled-up and can provide high yields. The resulting fluropolymers werefound to be effective in imparting oleophobic properties to polymerssuch as PTFE, and accordingly provide methodology for improving the oilrepellency of filter materials such as membranes comprising PTFE.Moreover, the fluoropolymer compositions of the disclosure are useful inimparting good hydrophobic and oleophobic properties to materials, whileallowing the passage of air through the materials. Coated materials suchas porous membranes and fabrics, including woven and nonwoven fibrousmaterials, which often are utilized within a filter structure, areuseful in applications where resistance to penetration by water, oil, orlipid emulsions ae desired. These applications include breathablefabrics and gas vents or filters. Gas vents or filters are often used toprotect electronic equipment in, for example, automotive, industrial,and medical device applications.

One key difficulty in the utilization of long linear perfluoro monomerslies in their limited solubility. In this improved process, wediscovered that the utilization of solvents having ester moieties in afree radical co-polymerization of (per)fluorine-containing monomers andfluorine-free monomers, such as acrylics, provides fluoropolymers inhigh yields and whose isolation is not fraught with the difficultiesdiscussed above, while at the same time providing a highly usefulfluoropolymer which can be easily dissolved in, for example,trifluorotoluene or butyl acetate. Advantageously, we also discoveredthat the utilization of certain acrylic monomers in conjunction withhigh fluorine-containing monomers in a polymerization with estersolvents provided fluoropolymers with high fluorine content, which wereexcellent polymers for oleophobic and hydrophobic modification of filtermembranes.

The resulting fluoropolymer reaction product can be diluted withadditional solvent(s), or can be directly used for uniform,oil-repellent modification of, for example, PTFE membranes. Theexperimental results show that the treated PTFE membrane can reach anoleophobic level of 6 or above, (according to the AATCC-118-1997 oilrepellency test method) with a low air flux loss.

DETAILED DESCRIPTION

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. As used in this specification and theappended claims, the term “or” is generally employed in its senseincluding “and/or” unless the content clearly dictates otherwise.

The term “about” generally refers to a range of numbers that isconsidered equivalent to the recited value (e.g., having the samefunction or result). In many instances, the term “about” may includenumbers that are rounded to the nearest significant figure.

Numerical ranges expressed using endpoints include all numbers subsumedwithin that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4 and5).

In a first aspect, the disclosure provides a process for preparing afluoropolymer, comprising the free-radical polymerization ofmono-ethylenically unsaturated monomers, which comprises combining:

-   -   a. about 60 to about 100 weight percent of a fluorine-containing        monoethylenically-unsaturated monomer; and    -   b. about 0 to about 40 weight percent of a fluorine-free        monoethylenically-unsaturated monomer;    -   under free-radical polymerization conditions,    -   in a solvent comprising at least one solvent having ester        moieties, wherein the total weight percent of a. and b. is 100,    -   and allowing the polymerization to proceed to a desired end        point.

In certain embodiments, the fluorine-containingmonoethylenically-unsaturated monomer (a.) will be present in thereaction mixture in an amount of about 70 to 100 weight percent, about80 to 100 weight percent, or about 90 to about 98 weight percent, withthe remainder comprising the fluorine-free monoethylenically-unsaturatedmonomer, and adding up to 100 weight percent total ofmonoethylenically-unsaturated monomers. In further embodiments, thefluorine-containing monoethylenically-unsaturated monomers and thefluorine-free monoethylenically-unsaturated monomers will be devoid ofhalogen atoms chosen from bromo, chloro, and iodo. In furtherembodiments, the fluorine-free monoethylenically-unsaturated monomersare free of cycloaliphatic groups and aryl groups. In furtherembodiments, the fluorine-free monoethylenically-unsaturated monomersare free of nitrogen atoms.

The process of the disclosure is typically conducted at temperatures ator above room temperature. In certain embodiments, the polymerization isconducted at a temperature of from about 20° C. to about 150° C., orabout 40° C. to about 120° C.

In this disclosure, free-radical polymerization conditions are thoseconditions which provide sufficient free-radical flux in the reactionmixture to effect polymerization of the fluorine-containing andfluorine-free monoethylenically-unsaturated monomers. Such free radicalscan be generated in solution via application of appropriate thermal orirradiation conditions such as, for example, ultraviolet radiation orelectron beam radiation. Alternately, and advantageously, thefree-radical flux may be effected by initiators known to those skilledin the art of free-radical polymerization. In one embodiment, suchinitiators can be chosen from hydrogen peroxide, potassiumperoxydisulfate, ammonium peroxydisulfate, potassium persulfate, sodiumpersulfate, ammonium persulfate, dibenzoyl peroxide, lauryl peroxide,di-tertiary butyl peroxide, 2,2′-azobisisobutyronitrile (or2,2′-azobis(2-methylpropionitrile) also known as AIBN), t-butylhydroperoxide, azodiisobutylamidine hydrochloride, and benzoyl peroxide.

Thus, in another embodiment, the disclosure provides a process forpreparing a fluoropolymer, comprising the free-radical polymerization ofmono-ethylenically unsaturated monomers, which comprises combining:

-   -   a. about 60 to about 100 weight percent of a fluorine-containing        monoethylenically-unsaturated monomer;    -   b. about 0 to about 40 weight percent of a fluorine-free        monoethylenically-unsaturated monomer; under free-radical        polymerization conditions,    -   c. and at least one free-radical initiator,

in a solvent comprising at least one solvent having ester moieties,wherein the total weight percent of a. and b. is 100, and

allowing the polymerization to proceed to a desired end point.

In certain embodiments, the fluorine-containingmonoethylenically-unsaturated monomer is chosen from

-   -   (a) acrylates,    -   (b) (C1-C14 alkyl)acrylates, and    -   (c) vinyl esters,    -   having from about 3 to about 33 fluorine atoms.

In other embodiments, the fluorine-containingmonoethylenically-unsaturated monomer will have from 8 to 20 fluorineatoms. In other embodiments, the fluorine-containingmonoethylenically-unsaturated monomer is a perfluorinated alkane havingone olefinic (i.e., double) bond.

In one embodiment, the fluorine-containing monoethylenically-unsaturatedmonomer has the formula

-   -   wherein R is chosen from hydrogen or an alkyl group of up to 18        carbon atoms, and R¹ is chosen from groups of the formulae:

-   -   wherein m is 3, 5, 7, 9, 11, 13, or 15.

In another embodiment, the fluorine-containingmonoethylenically-unsaturated monomer is chosen from one or more ofperfluorooctyl ethylene, perfluorononyl ethylene, perfluorodecylethylene, perfluorododecyl ethylene, perfluorotetradecyl ethylene,perfluorohexadecyl ethylene, 2-(perfluorooctyl)ethyl acrylate,2-(perfluorononyl)ethyl acrylate, 2-(perfluorodecyl)ethyl acrylate,2-(perfluorododecyl)ethyl acrylate, 2-(perfluorotetradecyl)ethylacrylate, 2-(perfluorohexadecyl)ethyl acrylate, 2-(perfluorooctyl)ethylmethacrylate, 2-(perfluorononyl)ethyl methacrylate,2-(perfluorodecyl)ethyl methacrylate, 2-(perfluorododecyl)ethylmethacrylate, 2-(perfluorotetradecyl)ethyl methacrylate,2-(perfluorohexadecyl)ethyl methacrylate, and the like.

The CAS numbers for exemplary fluorine-containingmonoethylenically-unsaturated monomers are listed in the followingtable:

Chemical Abstract Chemical Name Number (CAS No.)1H,1H,2H-heptadecafluoro-1-decene 21652-58-4 Perfluorodecyl ethylene30389-25-4 (Perfluorododecyl)ethylene 67103-05-31H,1H,2H,2H-heptadecafluorodecyl 1996-88-9 methacrylate1H,1H,2H,2H-Heptadecafluorodecyl 27905-45-9 Acrylate1,1,2,2-Tetrahydroperfluorotetradecyl 34395-24-9 acrylate1,1,2,2-Tetrahydroperfluorododecyl 2144-54-9 methacrylate2-(Perfluoroalkyl)ethyl methacrylate 65530-66-7 Perfluoroalkyl ethylacrylate 65605-70-1 Perfluoroalkyl ethylene 97659-47-4 Perfluorooctylethylene 21652-58-4 perfluorotetradecyl ethylene 97659-47-72-(perfluorooctyl)ethyl acrylate 17527-29-6 2-(perfluorododecyl)ethylacrylate 17741-60-5 2-(perfluorooctyl)ethyl methacrylate 1996-88-92-(perfluorododecyl)ethyl methacrylate 6014-75-12-(perfluorotetradecyl)ethyl methacrylate 2144-54-92-(perfluorooctyl)ethyl acrylate 27905-45-9

In another embodiment, the fluorine-containingmonoethylenically-unsaturated monomer is chosen from 2-(perfluorooctyl)ethyl acrylate, perfluorononyl ethyl acrylate, perfluorododecyl ethylacrylate, perfluorotetradecyl ethyl acrylate, perfluorohexadecyl ethylacrylate, perfluorooctyl ethyl methacrylate, perfluorononyl ethylacrylate methyl methacrylate, perfluorododecyl ethyl methacrylate, andperfluoro hexadecyl ethyl methacrylate.

In another embodiment, the fluorine-containingmonoethylenically-unsaturated monomer comprises, consists, or consistsessentially of 2-(perfluorooctyl) ethyl acrylate.

In one embodiment, the fluorine-free monoethylenically-unsaturatedmonomer is chosen from compounds of the formulae

-   -   wherein each R is independently chosen from hydrogen or an alkyl        group of up to 18 carbon atoms.

Compounds of Formula (A) will be recognized as representing acrylatesand (alkyl)acrylates and compounds of Formula (B) will be recognized asrepresenting certain vinyl compounds, i.e., vinyl esters.

In another embodiment, the fluorine-free monoethylenically-unsaturatedmonomer is chosen from compounds having an olefinic double bond, incertain cases attached directly attached to an aromatic ring. Examplesof such compounds are styrene and α-methyl styrene. Alternatively, theolefinic double bond may be substituted with an alkoxycarbonyl groupsuch as the case with di-n-butyl maleate. In other embodiments, thefluorine-free monoethylenically-unsaturated monomer may be those vinyland acrylate compounds having one or more nitrogen atoms, such ashydroxyethyl acrylamide. In other embodiments, the fluorine-freemonoethylenically-unsaturated monomers and/or the fluorine-containingmonoethylenically-unsaturated monomers exclude those monomers containinga nitrogen atom.

In one embodiment, the fluorine-free monoethylenically-unsaturatedmonomer is chosen from methyl acrylate, methyl methacrylate, ethylacrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate,isobutyl acrylate, isobutyl methacrylate, ethylhexyl acrylate,ethylhexyl methacrylate, octyl acrylate, octyl methacrylate, styrene,α-methyl styrene, glycidyl methacrylate, alkyl crotonates, vinylacetate, vinyl caprylate, di-n-butyl maleate, di-octylmaleate,hydroxyethyl acrylamide, hydroxypropyl methyl acrylamide, and the like.

In one embodiment, the fluorine-free monoethylenically-unsaturatedmonomer is chosen from methyl methacrylate, ethyl methacrylate, propylmethacrylate, butyl methacrylate, vinyl acetate, vinyl butyrate, andvinyl caprylate.

As used herein, the “solvent having ester moieties” is understood to bea solvent having at least one moiety such as

within the molecule.

In one embodiment, the solvent having ester moieties is chosen fromcompounds of the formula:

wherein R¹ and R² are independently chosen from (a) C₁-C₁₂ alkyl, and(b) C₁-C₁₂ alkyl groups substituted one or more times with a groupchosen from halo, nitro, cyano, and alkoxy.

In one embodiment, the solvent having ester moieties is chosen frommethyl acetate, ethyl acetate, propyl acetate, and butyl acetate.

As noted above, the fluoropolymers disclosed herein are particularlysuitable for coating porous polymeric membranes, in particular thosewhich are hydrophobic, such as PTFE. The physical properties of thefluoropolymers of the disclosure enable their dissolution in appropriatesolvents and application to such porous polymeric membranes, and in sodoing, at least partially coat the membrane and consequently enhance theoverall oleophobic and hydrophobic qualities of the membrane.Additionally, the fluoropolymers of the disclosure can be used to coatwoven and non-woven fibrous materials, for example such materials thatare used in textiles and filter materials, thus enhancing the oleophobicand hydrophobic qualities of such materials. Accordingly, in a furtheraspect, the disclosure provides a fluoropolymer prepared by the processof the disclosure. In a further embodiment, the disclosure provides afluoropolymer which is the free radical polymerization product ofmonomers comprising:

-   -   a. fluorine-containing monoethylenically-unsaturated monomers        chosen from: 2-(perfluorooctyl) ethyl acrylate, perfluorononyl        ethyl acrylate, perfluorododecyl ethyl acrylate,        perfluorotetradecyl ethyl acrylate, perfluorohexadecyl ethyl        acrylate, perfluorooctyl ethyl methacrylate, perfluorononyl        ethyl acrylate methyl methacrylate, perfluorododecyl ethyl        methacrylate, and perfluoro hexadecyl ethyl methacrylate;    -   b. and fluorine-free monoethylenically unsaturated monomers        chosen from: methyl acrylate, methyl methacrylate, ethyl        acrylate, ethyl methacrylate, butyl acrylate, butyl        methacrylate, isobutyl acrylate, isobutyl methacrylate,        ethylhexyl acrylate, ethylhexyl methacrylate, octyl acrylate,        octyl methacrylate, styrene, α-methyl styrene, glycidyl        methacrylate, alkyl crotonates, vinyl acetate, vinyl caprylate,        di-n-butyl maleate, and di-octylmaleate.

In this aspect, the monomers utilized in the preparation of thefluoropolymers may comprise, consist of, or consist essentially of therecited monomers above. As a general convention throughout thisdisclosure, the process, the composition as described, or an ingredientor component thereof, that is said to “consist essentially of” a groupof specified ingredients or materials refers to a composition thatcontains the specified ingredients or materials with not more than a lowor insignificant amount of other ingredients or materials, e.g., notmore than 5, 2, 1, 0.5, 0.1, or 0.05 weight percent of other ingredientsor materials.

In certain embodiments of the process and compositions of thedisclosure, the monomers are devoid of species containing halogen atomsother than fluorine, i.e., chloro, bromo, or iodo. As used herein,“devoid” refers to a quantity which is not more than 2, 1, 0.5, 0.1, or0.5 weight percent, based on the total weight of monomers utilized.

In one embodiment, the fluoropolymers of the invention are utilized tocoat filter materials, i.e., components of filters such as vent filters.Thus, in another aspect, the disclosure provides filters havingoleophobic and hydrophobic filtration capabilities. To perform afiltration function, such filters include the filter membranes of thedisclosure, which are responsible for removing unwanted material from agas which passes through the filter. The membranes within the filtersmay, as required, be in the form of a flat sheet, which may be wound(e.g., spirally), flat, pleated, or disc-shaped. The filter membrane mayalternatively be in the form of a hollow fiber. The filter containingthe membrane may be contained within a housing or otherwise supported sothat the gas which is being filtered enters through a filter inlet andis required to pass through the filter and membrane before passingthrough the filter outlet.

The membranes of the disclosure can be constructed of a porous structurethat has average pore sizes that can be selected based on the use of thefilter, i.e., the type of filtration performed by the filter. Typicalpore sizes are in the micron or sub-micron range, such as from about0.001 micron to about 10 μm. Membranes with average pore size of fromabout 0.002 to about 0.1 micron are sometimes classified as ultrafiltermembranes. Membranes with pore sizes between about 0.1 and 50 μm aresometimes referred to as microporous membranes. In one embodiment, themembranes comprise hydrophobic and/or oleophobic polymers. In oneembodiment, the membranes comprise poly(tetrafluorethylene), i.e., PTFE.

Thus, in another aspect, the disclosure provides a membrane having atleast a partial coating of a fluoropolymer thereon, wherein the membranehas an air flux of no less than 0.1 L/minute per 0.5024 cm² at apressure of 10 KPa, and an oil rating according to AATCC Test method228-1997 of greater than about 6. In one embodiment, the membrane is apoly(tetrafluoroethylene). In another embodiment, the membrane exhibitsan oil rating of about 7 to about 8. In another embodiment, the membraneis coated with the fluoropolymer prepared by the first aspect as setforth above.

In one embodiment, the filters and the membranes of the invention areuseful as vent filters. As noted above, vent filters are utilized inmany electronic devices, both to allow for internal and externalpressures to equalize, as well as to protect internal components fromdeleterious effects potentially brought on by exposure to water vaporand oil vapor. As provided by the present disclosure, the fluoropolymersare useful in providing a coating to existing polymeric membranes toimprove the membrane's inherent hydrophobic and oleophobic performance.In particular, these fluoropolymers, dissolved in an appropriatesolvent, can be used to apply a coating to a polymeric membrane such asa poly(tetrafluroethylene) membrane, thereby improving its hydrophobicand oleophobic performance. As such, the fluoropolymer coated membranesof the disclosure are particularly useful in those end use applicationswhich require oleophobicity and hydrophobicity. Additionally, themembranes of the invention are suitable for forming part of a filter.Accordingly, in another aspect, the disclosure provides a filtercomprising the membranes of the disclosure as set forth herein.

Given the improved hydrophobic and oleophobic performance that thefluoropolymers of the invention impart to polymeric membranes such asPTFE, these fluoropolymer-coated membranes can be advantageouslyutilized to remove water and oily materials from a gaseous flow, such asair. Thus, in a further aspect, the disclosure provides a method forpurifying a gas, which comprises passing a gas in need of purificationthrough the filter of the disclosure. In one embodiment, thispurification comprises the removal of at least a portion of the waterand/or oil vapor in the gaseous stream.

The membranes of the invention, which are porous polymeric membraneswhich have been at least partially coated with the fluoropolymers of thedisclosure, can be readily prepared by dissolving the fluoropolymerprepared in the first aspect, and further dissolving the fluoropolymersolution with solvents such as ethyl acetate, toluene, trifluorotoluene,and monofluorochloroethane, and mixtures thereof. In one embodiment, thediluting solvent is trifluorotoluene. The solution containing thefluoropolymer can then be sprayed onto the membrane or the membranesimply physically dipped into the solution, and thereafter allowed todry or otherwise drive off the solvent, thereby leaving a highlyoleophobic and hydrophobic coating on the underlying porous polymermembrane. Woven and nonwoven fibrous materials may be coated in the samemanner. In this fashion, a porous polymeric membrane, such aspoly(tetrafluoroethylene) (PTFE) which is inherently somewhat oleophobicand hydrophobic, can be enhanced in such qualities and performance.

EXAMPLES

Air Flux Test

The air flux was measured by an in-house testing device. The results arereported in terms of air flux which represents the passage of air at arate of liters (L) per minute for an effective membrane area of 0.5024square centimeter test sample, at a pressure of 10 KPa. A Rotameterflowmeter was utilized to measure the resulting air flow.

Oil Rating Data

Oil rating data was obtained by AATCC Test Method 118-1997. In this testmethod, the higher the number, the better the oil repellency

Example 1

90 g of perfluorooctyl ethyl acrylate, 10 g of butyl methacrylate, 1 gof benzoyl peroxide (BPO) as an initiator, and 300 g of butyl acetate assolvent were added into a three-neck round-bottom flask fitted with amechanical stirrer under a nitrogen atmosphere. The above reagentmixture was heated to 70° C. and reacted for 16 hours.

When cooled to room temperature, the fluoropolymer was diluted withtrifluorotoluene as a diluent to a weight concentration of 3% as anoleophobic modification agent. A sheet of PTFE composite membrane madeby Donaldson Company (product model TX6522, size 6 cm*26 cm) was dippedit into the modification reagent for 3 minutes and then taken out forair drying overnight. Oleophobic and air flux data are shown in Table 1below.

Example 2

60 g of perfluorooctyl ethyl acrylate, 40 g of dodecyl methacrylate, 1 gof benzoyl peroxide (BPO) as an initiator, and 300 g of butyl acetate asa solvent was added into a three-neck round-bottom flask fitted with amechanical stirrer under a nitrogen atmosphere. The above reagentmixture was heated to 110° C. and reacted for 16 hours.

When cooled to room temperature, the fluoropolymer was diluted with amixture solvent of monofluorodichloroethane and butyl acetate 1:1 byweight to a weight concentration of 3% as an oleophobic modificationagent. A sheet of PTFE composite membrane made by Donaldson Company(product model TX6522, size 6 cm*26 cm) was dipped into the modifierreagent for 3 minutes. The coated membrane was taken out, air dried for1 hour, and then dried in an oven at 70° C. for 10 minutes. Oleophobicand air flux data are shown in Table 1 below.

Comparative Example 1—(Non-Ester Solvent)

An oleophobic fluoropolymer was prepared in accordance with the teachingin U.S. Pat. No. 5,342,434:

5.10 g of methylene di-p-phenyl diisocyanate and 20.56 g ofperfluoroalkyl ethyl alcohol (Zonyl BA-N, from DuPont) were placed in avial, heated to 120° C., and kept at 120° C. with stirring for 1 hourfollowed by heating to 190° C. for another hour. When cooled to roomtemperature, the reaction product became a brown solid. Meltingtemperature of the reaction product was in the range of 150° to 180° C.as determined by differential scanning calorimetry (DSC) at a rate of10° C. increase/minute in air. 1 gram of the reaction product wasdissolved in 20 grams of tetrahydrofuran at 50° C. Sheets of expandedporous PTFE (ePTFE) of pore size described further below (obtained fromW. L. Gore & Associates, Inc.) were dipped in the resulting solution fora time sufficient to produce a desired “add-on” amount of coating, i.e.,utilizing the coating methodology of Example 1. Oleophobic and air fluxdata are shown in Table 1 below.

Comparative Example 2 (Non-Ester Fluoropolymer Diluent)

An oleophobic fluoropolymer was prepared in accordance with the teachingin U.S. Pat. No. 9,168,472:

100 g of a compound having a linear fluoroalkyl group and represented bythe Formula (b-1) shown below, 0.1 g of azobisisobutyronitrile servingas a polymerization initiator, and 300 g sample of a solvent, “FSthinner” manufactured by Shin-Etsu Chemical Co., Ltd. (a conventionalfluoropolymer diluent; see U.S. Pat. No. 9,254,467) were put into aflask equipped with a nitrogen introducing tube, a thermometer, and astirrer. Nitrogen gas was introduced while stirring was performed at 70°C., and addition polymerization was thus allowed to proceed for 16hours. As a result, 80 g of a fluorine-containing polymer was obtained.The number average molecular weight of this polymer was 100,000. Awater/oil-repellent treatment liquid was prepared by diluting thefluorine-containing polymer with a diluting agent (“FS thinner”manufactured by Shin-Etsu Chemical Co., Ltd.) so that the concentrationof the fluorine-containing polymer was 3.0% by weight.

CH₂═CHCOOCH₂CH₂C₆F13  (Formula b-1)

TABLE 1 Data comparison before and after oleophobic treatment of PTFEcomposite membrane.* Oil Sample rating Air flux (L/min) Air flux lossrate Before oleophobic 0 0.15 / treatment Example 1 8 0.105~0.12 20%~30% Example 2 7 0.1125~0.1275 15%~25% Comparative 7  0.09~0.10530%~40% Example 1 *Representative data accumulated from other batchesprepared essentially according to the referenced examples.

Example 3—Weight Gain of Membrane Upon Coating

Using the methodology of the invention, the fluoropolymer compositionsof the invention were used to coat poly(tetrafluorethylene) (PTFE)membranes:

Membrane weight after Weight gain Weight gain oleophobic per Weight gainin per Membrane Membrane modification membrane percentage membraneSample area(cm²) weight (g) (g) weight(g/g) (%) area(g/cm²) 1# 75 0.51690.5252 0.0161 1.6057 0.0001 2# 0.5259 0.542 0.0306 3.0614 0.0002 3#0.5187 0.5437 0.0482 4.8197 0.0003

TABLE 2* Weight Proportion of Weight Air Flux Fluoronated Concentrationof Oleophbic Loss Sample Monomer Diluent Solvent Fluropolymer Level**Yield Rate 1 30% Ethyl acetate 2% 1 >92% 15-20% 2 40% Ethyl acetate 2%3 >92% 15-20% 3 50% Ethyl acetate 2% 4 >92% 15-20% 4 60% Ethyl acetate2% 6 >92% 15-20% 5 70% Ethyl acetate 2% 6 >92% 15-20% 6 ≥75%  A solventmixed with 2% ≥7 >92% 15-20% ethyl acetate and trifluorotoluene or ethylacetate and monofluoro- chlorethane 7 ≥98%  Trifluorotoluene 2% 8 >92%15-20% or monofluoro- chloroethane *The data in Table 2 isrepresentative of accumulated data from multiple experiments.**According to AATCC Test Method 118-1997

As shown in Table 2 above, as the usage of fluorine-containing monomersapproaches 100 weight percent, the resulting fluoropolymer will requireincreasing amounts of fluoride-containing solvent, such astrifluorotoluene or monofluoro-chloroethane in order to dissolve intosolution.

Example 5—Soaking Experiment

In order to verify the stability of the fluoropolymer-coated membranesof the disclosure, the following experiment was conducted:

First, 15 identical poly(tetrafluorethylene) (PTFE) samples which hadbeen treated with the (oleophobic) fluoropolymer of the disclosure(Example 1) and three PTFE samples without this oleophobic treatment,respectively marked as Nos. 1 to No. 18. The samples were tested torecord the oleophobic grade before soaking. Next, three differentsoaking solutions, namely water, ethanol and butyl acetate were utilizedand three oleophobic treated PTFE samples and one untreated PTFE whichwas used as a comparison for each solution. Each sample was taken outafter soaking for 48 hours, dried, and subjected to oleophobic testing.Each sample was characterized by FT-IR and Ultraviolet spectra andresults are shown in Table 3:

TABLE 3 Oleophobic Soaking for 48 hours Soaking Level Before Oleophobicliquid Soaking FT-IR UV Level Water 7 No extra peaks* No extra peaks 7Ethyl 7 No extra peaks  No extra peaks 7 Alcohol Butyl 7 No extra peaks No extra peaks 7 Acetate *No extra peaks were observed in the FT-IR andUV spectra after the soaking experiments, evidencing that under theconditions of the treatments, the membrane and its coating were stable,i.e., did not degrade to form other species which would have otherwisebeen identifiable by infrared or ultraviolet spectral analysis.

Aspects

In a first aspect, the disclosure provides a process for preparing afluoropolymer, comprising the free-radical polymerization ofmono-ethylenically unsaturated monomers, which comprises combining:

-   -   a. about 60 to about 100 weight percent of a fluorine-containing        monoethylenically-unsaturated monomer; and    -   b. about 0 to about 40 weight percent of a fluorine-free        monoethylenically-unsaturated monomer;    -   under free-radical polymerization conditions,    -   in a solvent comprising at least one solvent having ester        moieties, wherein the total weight percent of a. and b. is 100,    -   and allowing the polymerization to proceed to a desired end        point.

In a second aspect, the disclosure provides the process of the firstaspect, wherein the free radical polymerization conditions comprise theaddition of at least one free-radical initiator.

In a third aspect, the disclosure provides the process of the secondaspect, wherein the free-radical initiator is a compound chosen fromhydrogen peroxide, potassium peroxydisulfate, ammonium peroxydisulfate,potassium persulfate, sodium persulfate, ammonium persulfate, dibenzoylperoxide, lauryl peroxide, di-tertiary butyl peroxide,2,2′-azobisisobutyronitrile, t-butyl hydroperoxide, azodiisobutylamidinehydrochloride, and benzoyl peroxide.

In a fourth aspect, the disclosure provides the process of any one ofthe first to third aspects, wherein the fluorine-containingmonoethylenically-unsaturated monomer is chosen from

-   -   (a) acrylates,    -   (b) (C₁-C₁₄ alkyl)acrylates, and    -   (c) vinyl esters,

having from about 3 to about 33 fluorine atoms.

In a fifth aspect, the disclosure provides the process of any one of thefirst through the fourth aspects, wherein the fluorine-containingmonoethylenically-unsaturated monomer has the formula

-   -   wherein R is chosen from hydrogen or an alkyl group of up to 18        carbon atoms, and R¹ is chosen from groups of the formulae:

-   -   wherein m is 3, 5, 7, 9, 11, 13, or 15.

In a sixth aspect, the invention provides the process of any one of thefirst through the fifth aspects, wherein the fluorine-containingmonoethylenically-free monomer is chosen from one or more ofperfluorooctylethylene, perfluorononylethylene,perfluorododecylethylene, perfluorotetra-decylethylene,perfluorohexadecylethylene, perfluorooctyl ethyl acrylate,perfluorononyl ethyl base acrylate, perfluorododecyl ethyl acrylate,perfluoro tetradecyl ethyl methacrylate, perfluoro tetradecyl ethylacrylate, perfluoro hexadecyl ethyl acrylate, perfluorooctylethylmethacrylate, perfluorononylethyl methacrylate, perfluorododecylethylmethacrylate, and perfluorohexadecylethyl methacrylate.

In a seventh aspect, the disclosure provides the process of any one ofthe first through sixth aspects, wherein the fluorine-containingmonoethylenically-unsaturated monomer is chosen from 2-(perfluorooctyl)ethyl acrylate, perfluorononyl ethyl acrylate, perfluorododecyl ethylacrylate, perfluorotetradecyl ethyl acrylate, perfluorohexadecyl ethylacrylate, perfluorooctyl ethyl methacrylate, perfluorononyl ethylacrylate methyl methacrylate, perfluorododecyl ethyl methacrylate,perfluoro hexadecyl ethyl methacrylate.

In an eighth aspect, the disclosure provides the process of any one ofthe first through seventh aspects, wherein the fluorine-containingmonoethylenically-unsaturated monomer is 2-(perfluorooctyl) ethylacrylate.

In a ninth aspect, the disclosure provides the process of any one of thefirst through the eighth aspects wherein the fluorine-freemonoethylenically-unsaturated monomer is chosen from compounds of theformulae

-   -   wherein each R is independently chosen from hydrogen or an alkyl        group of up to 18 carbon atoms.

In a tenth aspect, the disclosure provides the process of any one of thefirst through ninth aspects, wherein the fluorine-freemonoethylenically-unsaturated monomer is chosen from methyl acrylate,methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate,butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, ethylhexylacrylate, ethylhexyl methacrylate, octyl acrylate, octyl methacrylate,styrene, α-methyl styrene, glycidyl methacrylate, alkyl crotonates,vinyl acetate, vinyl caprylate, di-n-butyl maleate, di-octylmaleate,hydroxyethyl acrylamide, hydroxypropyl methyl acrylamide.

In an eleventh aspect, the disclosure provides the process of any one ofthe first through tenth aspects, wherein the fluorine-freemonoethylenically-unsaturated monomer is chosen from methylmethacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate, vinyl acetate, vinyl butyrate, and vinyl caprylate.

In a twelfth aspect, the disclosure provides the process of any one ofthe first through eleventh aspects, wherein the solvent having estermoieties is chosen from compounds of the formula:

-   -   wherein R¹ and R² are independently chosen from (a) C₁-C₁₂        alkyl, and (b) C₁-C₁₂ alkyl groups substituted one or more times        with a group chosen from halo, nitro, cyano, and alkoxy.

In a thirteenth aspect, the disclosure provides the process of any oneof the first through the twelfth aspects, wherein the solvent havingester moieties is chosen from methyl acetate, ethyl acetate, propylacetate, and butyl acetate.

In a fourteenth aspect, the disclosure provides a fluoropolymer preparedby the process of any one of the first through the thirteenth aspects.

In a fifteenth aspect, the disclosure provides a fluoropolymer which isthe free radical polymerization product of monomers comprising,consisting, or consisting essentially of:

-   -   a. fluorine-containing monoethylenically-unsaturated monomers        chosen from; 2-(perfluorooctyl) ethyl acrylate, perfluorononyl        ethyl acrylate, perfluorododecyl ethyl acrylate,        perfluorotetradecyl ethyl acrylate, perfluorohexadecyl ethyl        acrylate, perfluorooctyl ethyl methacrylate, perfluorononyl        ethyl acrylate methyl methacrylate, perfluorododecyl ethyl        methacrylate, and perfluoro hexadecyl ethyl methacrylate; and    -   b. fluorine-free monoethylenically unsaturated monomers chosen        from: methyl acrylate, methyl methacrylate, ethyl acrylate,        ethyl methacrylate, butyl acrylate, butyl methacrylate, isobutyl        acrylate, isobutyl methacrylate, ethylhexyl acrylate, ethylhexyl        methacrylate, octyl acrylate, octyl methacrylate, styrene,        α-methyl styrene, glycidyl methacrylate, alkyl crotonates, vinyl        acetate, vinyl caprylate, di-n-butyl maleate, and        di-octylmaleate.

In a sixteenth aspect, the disclosure provides a membrane having atleast a partial coating of a fluoropolymer thereon, wherein the membraneexhibits an oil rating according to AATC Test Method 118-1997 of greaterthan or equal to 6, wherein the fluoropolymer is devoid of halogen atomschosen from chlorine, bromine, and iodine.

In a seventeenth aspect, the disclosure provides a membrane having atleast a partial coating of fluoropolymer thereon, wherein thefluoropolymer is prepared by the process of any one of the first throughthe thirteenth aspects, wherein the membrane exhibits an oil ratingaccording to AATC Test Method 118-1997 of greater than or equal to 6,and wherein the fluoropolymer is devoid of halogen atoms chosen fromchlorine, bromine, and iodine.

In an eighteenth aspect, the disclosure provides the membrane of thesixteenth or seventeenth aspects, wherein the fluoropolymer is preparedfrom fluorine-containing monoethylenically-unsaturated monomers chosenfrom;

-   -   a. 2-(perfluorooctyl) ethyl acrylate, perfluorononyl ethyl        acrylate, perfluorododecyl ethyl acrylate, perfluorotetradecyl        ethyl acrylate, perfluorohexadecyl ethyl acrylate,        perfluorooctyl ethyl methacrylate, perfluorononyl ethyl acrylate        methyl methacrylate, perfluorododecyl ethyl methacrylate, and        perfluoro hexadecyl ethyl methacrylate;        and fluorine-free monoethylenically unsaturated monomers chosen        from:    -   b. methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl        methacrylate, butyl acrylate, butyl methacrylate, isobutyl        acrylate, isobutyl methacrylate, ethylhexyl acrylate, ethylhexyl        methacrylate, octyl acrylate, octyl methacrylate, styrene,        α-methyl styrene, glycidyl methacrylate, alkyl crotonates, vinyl        acetate, vinyl caprylate, di-n-butyl maleate, and        di-octylmaleate.

In a nineteenth aspect, the disclosure provides the membrane of theeighteenth aspect, wherein the fluorine-containingmonoethylenically-unsaturated monomer comprises, consists, or consistsessentially of 2-(perfluorooctyl) ethyl acrylate, and the fluorine-freemonoethylenically-unsaturated monomer is chosen from methylmethacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate, vinyl acetate, vinyl butyrate, and vinyl caprylate.

In a twentieth aspect, the disclosure provides the membrane of theeighteenth or nineteenth aspect, wherein the exhibits an oil ratingaccording to AATC Test Method 118-1997 of about 7 to 8.

In a twenty-first aspect, the disclosure provides the membrane of thetwentieth aspect, wherein the membrane has an air flux of no less than0.1 liters per minute when passed through a membrane sample having aneffective area of about 0.5024 cm².

In a twenty-second aspect, the disclosure provides the membrane of thetwentieth or twenty-first aspect, wherein the membrane exhibits an airflux loss rate of no greater than 30 percent, when compared to a likemembrane having no fluoropolymer coating.

In a twenty-third aspect, the disclosure provides the membrane of anyone of the eighteenth through twenty second aspects, wherein themembrane is a poly(tetrafluoroethylene).

In a twenty-fourth aspect, the disclosure provides a woven or nonwovenfibrous material coated with the fluoropolymer prepared by the processof any one of the first through the thirteenth aspects.

In a twenty-fourth aspect, the disclosure provides a filter comprisingthe membrane or fibrous material of any one of the sixteenth through thetwenty-fourth aspects.

In a twenty-fifth aspect, the disclosure provides the filter of thetwenty-fourth aspect, wherein the filter is a vent filter.

In a twenty-sixth aspect, the disclosure provides a method for purifyinga gas, which comprises passing a gas in need of purification through thefilter of the twenty-fourth or twenty-fifth aspect.

Having thus described several illustrative embodiments of the presentdisclosure, those of skill in the art will readily appreciate that yetother embodiments may be made and used within the scope of the claimshereto attached. Numerous advantages of the disclosure covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many respects, onlyillustrative. The disclosure's scope is, of course, defined in thelanguage in which the appended claims are expressed.

What is claimed is:
 1. A process for preparing a fluoropolymer,comprising the free-radical polymerization of mono-ethylenicallyunsaturated monomers, which comprises combining: a. about 60 to about100 weight percent of a fluorine-containingmonoethylenically-unsaturated monomer; and b. about 0 to about 40 weightpercent of a fluorine-free monoethylenically-unsaturated monomer; underfree-radical polymerization conditions, in a solvent comprising at leastone solvent having ester moieties, wherein the total weight percent ofa. and b. is 100, and allowing the polymerization to proceed to adesired end point.
 2. The process of claim 1, wherein the free radicalpolymerization conditions comprise the addition of at least onefree-radical initiator.
 3. The process of claim 2, wherein thefree-radical initiator is a compound chosen from hydrogen peroxide,potassium peroxydisulfate, ammonium peroxydisulfate, potassiumpersulfate, sodium persulfate, ammonium persulfate, dibenzoyl peroxide,lauryl peroxide, di-tertiary butyl peroxide,2,2′-azobisisobutyronitrile, t-butyl hydroperoxide, azodiisobutylamidinehydrochloride, and benzoyl peroxide.
 4. The process of claim 1, whereinthe fluorine-containing monoethylenically-unsaturated monomer is chosenfrom (a) acrylates, (b) (C₁-C₁₄ alkyl)acrylates, and (c) vinyl esters,having from about 3 to about 33 fluorine atoms.
 5. The process of claim1, wherein the fluorine-containing monoethylenically-unsaturated monomerhas the formula

wherein R is chosen from hydrogen or an alkyl group of up to 18 carbonatoms, and R¹ is chosen from groups of the formulae:

wherein m is 3, 5, 7, 9, 11, 13, or
 15. 6. The process of claim 1,wherein the fluorine-containing monoethylenically-unsaturated monomer ischosen from one or more of perfluorooctylethylene,perfluorononylethylene, perfluorododecylethylene,perfluorotetra-decylethylene, perfluorohexadecylethylene, perfluorooctylethyl acrylate, perfluorononyl ethyl base acrylate, perfluorododecylethyl acrylate, perfluoro tetradecyl ethyl methacrylate, perfluorotetradecyl ethyl acrylate, perfluoro hexadecyl ethyl acrylate,perfluorooctylethyl methacrylate, perfluorononylethyl methacrylate,perfluorododecylethyl methacrylate, and perfluorohexadecylethylmethacrylate.
 7. The process of claim 1, wherein the fluorine-containingmonoethylenically-unsaturated monomer is chosen from 2-(perfluorooctyl)ethyl acrylate, perfluorononyl ethyl acrylate, perfluorododecyl ethylacrylate, perfluorotetradecyl ethyl acrylate, perfluorohexadecyl ethylacrylate, perfluorooctyl ethyl methacrylate, perfluorononyl ethylacrylate methyl methacrylate, perfluorododecyl ethyl methacrylate, andperfluoro hexadecyl ethyl methacrylate.
 8. The process of claim 1,wherein the fluorine-containing monoethylenically-unsaturated monomercomprises 2-(perfluorooctyl) ethyl acrylate.
 9. The process of claim 1,wherein the fluorine-free monoethylenically-unsaturated monomer ischosen from compounds of the formulae

wherein each R is independently chosen from hydrogen or an alkyl groupof up to 18 carbon atoms.
 10. The process of claim 1, wherein thefluorine-free monoethylenically-unsaturated monomer is chosen frommethyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butyl methacrylate, isobutyl acrylate,isobutyl methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate,octyl acrylate, octyl methacrylate, styrene, a-methyl styrene, glycidylmethacrylate, alkyl crotonates, vinyl acetate, vinyl caprylate,di-n-butyl maleate, di-octylmaleate, hydroxyethyl acrylamide,hydroxypropyl methyl acrylamide.
 11. The process of claim 1, wherein thefluorine-free monoethylenically-unsaturated monomer is chosen frommethyl methacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate, vinyl acetate, vinyl butyrate, and vinyl caprylate. 12.The process of claim 1, wherein the solvent having ester moieties ischosen from compounds of the formula:

wherein R¹ and R² are independently chosen from (a) C₁-C₁₂ alkyl, and(b) C₁-C₁₂ alkyl groups substituted one or more times with a groupchosen from halo, nitro, cyano, and alkoxy.
 13. The process of claim 1,wherein the solvent having ester moieties is chosen from methyl acetate,ethyl acetate, propyl acetate, and butyl acetate.
 14. A fluoropolymerprepared by the process of claim
 1. 15. The fluoropolymer of claim 14,which is the free radical polymerization product of monomers comprising:a. fluorine-containing monoethylenically-unsaturated monomers chosenfrom; 2-(perfluorooctyl) ethyl acrylate, perfluorononyl ethyl acrylate,perfluorododecyl ethyl acrylate, perfluorotetradecyl ethyl acrylate,perfluorohexadecyl ethyl acrylate, perfluorooctyl ethyl methacrylate,perfluorononyl ethyl acrylate methyl methacrylate, perfluorododecylethyl methacrylate, and perfluoro hexadecyl ethyl methacrylate; b. andfluorine-free monoethylenically unsaturated monomers chosen from: methylacrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butylacrylate, butyl methacrylate, isobutyl acrylate, isobutyl methacrylate,ethylhexyl acrylate, ethylhexyl methacrylate, octyl acrylate, octylmethacrylate, styrene, α-methyl styrene, glycidyl methacrylate, alkylcrotonates, vinyl acetate, vinyl caprylate, di-n-butyl maleate, anddi-octylmaleate.
 16. A membrane comprising at least a partial coating offluoropolymer thereon, wherein the membrane exhibits an oil ratingaccording to AATC Test Method 118-1997 of greater than or equal to 6,and wherein the fluoropolymer is devoid of halogen atoms chosen fromchlorine, bromine, and iodine.
 17. The membrane of claim 16, wherein thefluoropolymer is prepared from fluorine-containingmonoethylenically-unsaturated monomers chosen from; a.2-(perfluorooctyl) ethyl acrylate, perfluorononyl ethyl acrylate,perfluorododecyl ethyl acrylate, perfluorotetradecyl ethyl acrylate,perfluorohexadecyl ethyl acrylate, perfluorooctyl ethyl methacrylate,perfluorononyl ethyl acrylate methyl methacrylate, perfluorododecylethyl methacrylate, and perfluoro hexadecyl ethyl methacrylate; andfluorine-free monoethylenically unsaturated monomers chosen from: b.methyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butyl methacrylate, isobutyl acrylate,isobutyl methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate,octyl acrylate, octyl methacrylate, styrene, a-methyl styrene, glycidylmethacrylate, alkyl crotonates, vinyl acetate, vinyl caprylate,di-n-butyl maleate, and di-octylmaleate.
 18. The membrane of claim 16,wherein the membrane exhibits an oil rating according to AATC TestMethod 118-1997 of about 7 to
 8. 19. The membrane of claim 18, whereinthe membrane has an air flux of no less than 0.1 liters per minute whenpassed through a membrane sample having an effective area of about0.5024 cm².
 20. The membrane of claim 19, wherein the membrane exhibitsan air flux loss rate of no greater than 30 percent, when compared to alike membrane having no fluoropolymer coating.
 21. A filter comprisingthe membrane of claim 1.